Respirators have been used for many years to aid the breathing cycles of patients in a variety of situations. Typically, a respirator may employ controls capable of regulating the supply of breathable gas to a patient mask during both inspiratory and expiratory phases.
In the U.S. patent to Sundblom et al. U.S. Pat. No. 3,669,108 a respirator is described to provide both volume and pressure breathing cycle control. In a volume cycled control, a predetermined volume of gas is supplied to a patient during a particular time period while in a pressure cycled control the inspiratory gas flow is terminated when a predetermined alveolar or airway pressure occurs.
In the Sundblom patent the illustrated respirator provides a variety of different operational conditions. For example, with one type of control one may automatically maintain a preset ratio of inspiratory time to expiratory time. Other controls permit an independent setting of the inspiratory time or tidal volume, manual override and a flow pattern adjustment to determine limits for the initial and terminal gas flows.
The automatic control over ratio of inspiratory time to expiratory time is achieved in the Sundblom patent respirator by allowing the pressure in a gas chamber to build up during the inspiratory cycle. The final gas pressure attained then becomes a function of the inspiratory time and the volume of gas stored during the inspiratory cycle. The final gas pressure attained then becomes a function of the inspiratory time and the volume of gas stored during this period is controllably discharged to set an expiratory time period.